Separate parsing from configuration for type safety

What does this MR do?

Declares types for Configuration fields to improve type-safety in the DAST repository.

What are the relevant issue numbers?

gitlab-org/gitlab#218814 (closed), specifically this was discussed in !252 (comment 395122971).

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer